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Surging idle from a 350 TBI

This is a discussion on Surging idle from a 350 TBI within the Technical / Maintenance forums, part of the General Discussion category; . 1994 K1500 Suburban with a TBI 350 I just bought this thing and it has a surging idle. It ...

  1. #1
    Big Daddy 1-8-4-3-6-5-7-2's Avatar
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    Surging idle from a 350 TBI


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    1994 K1500 Suburban with a TBI 350

    I just bought this thing and it has a surging idle. It idles fine when the engine first starts, then starts surging after a couple minutes. It smells like it's running real rich too. The idle will vary several hundred RPM at a frequency of a couple seconds between surges. It sounds like the engine almost dies then springs back to life. The vehicle ran fine then this problem just started.

    The check engine light is off.


    If I unplug something, like the wiring connector for the MAP sensor, the engine will idle fine, but still smells rich.

    The previous owner replaced several parts instead of testing them, then brought it to a GM dealer who couldn't fix it either.

    Parts he replaced, none of them changed anything:

    IAC
    TPS
    02 sensor
    EGR (from a junkyard, so it may have ben trash too?)
    EGR solenoid
    Cap, rotor, plugs
    MAP sensor
    Temp sensor
    PCM
    Spark module? (he said it's inside the distributor)
    Checked extensively for vacuum leaks



    I checked the alternator voltage and it was 14.3 volts. The truck has an MSD coil, but I don't know how to check for 13.8 volts on the red 10 gauge wire.


    I checked the timing also... it's right on 0*.

    I went through a tall can of starting fluid, spraying on all vacuum hoses, throttle body and gaskets, and intake gaskets and found no leaks.

    The engine has rapid fire plugs and an MSD coil... I'm going to check them when I get some time. The engine seems to have a miss when revved up too.


    Where should I go from here?




  2. #2
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    Re: Surging idle from a 350 TBI

    Damn dude....you checked all the right things. The IAC was replaced....was it set correctly?

    Keep in mind..just b/c he said he replaced all that doesn't mean he did. Might wanna doublecheck.
    1993 Chevy 5.7 tbi Z71, vortec heads, GMPP tbi vortec intake, 2" TBI and bored intake, TBI spacer, Comp Cams 08-409-8 XTREME 4x4 roller cam conversion, Hedman full length headers, 3" body lift, 33" tires, Paxton Performance tune (aka 94Blazer383), 4.10's, Hypertech powercharger, built 4l60e.

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    Big Daddy 1-8-4-3-6-5-7-2's Avatar
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    Re: Surging idle from a 350 TBI

    I haven't set the IAC yet. It still has the same problem as before the IAC was replaced, but I guess your right and I should set it anyway to eliminate that as a possible cause of the problem. I'm tempted to buy an ALDL cord to data log and send the data to someone who can "read" it.

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    Re: Surging idle from a 350 TBI

    Is ther any chance the guy you bought it from installed a cam and never told you about it? It sounds a lot like what mine was doing before I got a custom tune. He may have to tried to fix it by doing all the other stuff not knowing a tune was needed. Some guys are old school and might not know the PCM needs altering. Trust me, I was ready to get rid of my truck as well when it was doing that. I can't believe I made it through it b/c it's a huge PITA.

    Just brainstorming here....

    Like you mentioned....some datalogs would help
    Last edited by 93ChevyTBI; 09-25-2008 at 02:37 PM.
    1993 Chevy 5.7 tbi Z71, vortec heads, GMPP tbi vortec intake, 2" TBI and bored intake, TBI spacer, Comp Cams 08-409-8 XTREME 4x4 roller cam conversion, Hedman full length headers, 3" body lift, 33" tires, Paxton Performance tune (aka 94Blazer383), 4.10's, Hypertech powercharger, built 4l60e.

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    D to the MAX gr8shot's Avatar
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    Re: Surging idle from a 350 TBI

    man I feel your pain, I just bought a truck (the bottom one in my sig) and it's doing the exact same thing!! My only problem thats different from yours is that I have a charging problem. Gague shows about 9-10V when running. As soon as my multimeter I ordered arrives, I'll be busy.

    I've replaced plugs, wires, IAC valve, map sensor, and coolant temp sensor so far. Mine also runs good without the map sensor hooked up, and I also have no vac. leaks that I can find (also used a whole can of either to test).

    Lets make a deal, If you fix it, make sure you report back here so I know how to fix mine (or at least something else to try), and if I fix mine, I'll let you know!! LOL

    Here's some vids I took of what mine's doing...
    http://www.youtube.com/watch?v=PoX260u5AQ4
    http://www.youtube.com/watch?v=RjEptQ3hZkY
    http://www.youtube.com/watch?v=2Y-4lFnUH30

    Similar to what your truck is doin?
    04 Chevy LB7 Dmax. Some go fast goodies from Husker Diesel

    SOLD!!!! 07.5 Chevy LMM Dmax DPF Deleted, EFI Live from Husker Diesel & more 448hp/834tq

    1992 Chevy 1500 ECSB 4x4 350TBI 320k miles

    '95 Chevy 2500 ECLB swapped in a 95 12v cummins, NV4500, 2wd

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    Big Daddy 1-8-4-3-6-5-7-2's Avatar
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    Re: Surging idle from a 350 TBI

    I actually bought it from my brother. He lived in Va Beach and owned the Suburban for a while and it always ran fine. He then moved to Illinois and it ran fine for the whole trip. About a week after moving to Illinois it just started acting up. He doesn't take much time to diagnose problems... instead he just throws new parts at it, but none of the parts changed the problem for better or worse.

    I'm trying to learn as I go here... when I disconnect the wiring connector for the map sensor it will idle smooth, but smell real rich out the exhaust. He claims the cat has been gutted, I'm not sure if that contributes to the smell or not? Anyway, would disconnecting the map wiring put the computer into a limp mode where it would run on a predetermined table? And if so, would a rich condition possibly indicate a weak ignition system?



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    Re: Surging idle from a 350 TBI

    gr8shot - my idle is very similar to your's in the first video, but at a lower RPM and mine doesn't die except sometimes when I come up to stop light, just before the truck stops rolling. We'll keep each other posted.
    Last edited by 1-8-4-3-6-5-7-2; 09-25-2008 at 03:30 PM.

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    Re: Surging idle from a 350 TBI

    no cat will make it smell like it's running richer....mine does too. Small price to pay :)
    1993 Chevy 5.7 tbi Z71, vortec heads, GMPP tbi vortec intake, 2" TBI and bored intake, TBI spacer, Comp Cams 08-409-8 XTREME 4x4 roller cam conversion, Hedman full length headers, 3" body lift, 33" tires, Paxton Performance tune (aka 94Blazer383), 4.10's, Hypertech powercharger, built 4l60e.

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    Re: Surging idle from a 350 TBI

    .



    Here's an old video from over a year ago. The exhaust is fairly loud in the video making it hard to hear the surging. It sounds worse than in the video.




  10. #10
    D to the MAX gr8shot's Avatar
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    Re: Surging idle from a 350 TBI

    I got an idea for you. I'm following another thread where a guy is having almost the same problem you and I are. He tracked it down to his ICM. He took it in and it tested bad. Put a new one on, and it ran good. His truck friend two new ones however, but that seems to be his problem. I have never messed with a distributor before, and don't wanna f*** mine up, so either I'll have a mech. do it for me, or try other things, but if your comfortable takin yours off, it'd be something else to try, since there's not much left that can be bad!!
    04 Chevy LB7 Dmax. Some go fast goodies from Husker Diesel

    SOLD!!!! 07.5 Chevy LMM Dmax DPF Deleted, EFI Live from Husker Diesel & more 448hp/834tq

    1992 Chevy 1500 ECSB 4x4 350TBI 320k miles

    '95 Chevy 2500 ECLB swapped in a 95 12v cummins, NV4500, 2wd

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    Re: Surging idle from a 350 TBI

    Yeah, I've been watching that one also. I took mine apart earlier today and there was no grease on the module. I went and bought a new module and a stock coil to replace the MSD coil that was on there. It helped some... but the problem is still there.

    I found a thread on another forum where the guy had the same problem and a new fuel pump fixed it. I haven't checked my fuel pressure yet, but my brother said the guy at the Chevy dealer did check it and it was @ 13 psi. I will probably check it again to be sure.



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    Re: Surging idle from a 350 TBI

    ..



    Ok, so updated new info that I did not know about until now...

    While in Va Beach my brother brought the truck to shop to get new injectors put in. It ran worse when he got it back. He brough it back to be fixed, but it ran even worse again after he got it back.

    It almost seems neverending...

  13. #13
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    Re: Surging idle from a 350 TBI

    My 92 is acting the exact same way. It has 155,000 miles, I have too replaced the catalytic converter with no improvement at all. I will post anything I can find as well. This problem is very agrivating, no one I have talked to has ever herd anything like it. subscribed!

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    Re: Surging idle from a 350 TBI

    .

    Ok, I checked the fuel pressure today with one of the crappy gauges from Autozone. It was 15 PSI once when I checked it and 13 PSI the other times when I checked it.

    The headers have a couple very small leaks where they bolt to the heads. I plan to fix the leaks and get a new exhaust system bent for it.

    So, my next steps...

    1. Fix all leaks in exhaust system
    2. Buy the data cable and log some data and send to someone who can "read" it.
    3. Buy a few gallons of gas and a book of matches.



    .

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    Re: Surging idle from a 350 TBI

    with the map sensor disconnected the ecm substitutes a fixed value and controls fuel via tps sensor.
    O2 GMC RCSB 4.8L V8 AUTO 3.73's
    rebuilt/beefed up 4L60E(at 43,000miles)
    Matching Ninja 500r


  16. #16
    Big Daddy 1-8-4-3-6-5-7-2's Avatar
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    Re: Surging idle from a 350 TBI

    Yeah, and that's how the previous owner ran it all the time. It smelled like it was running very rich.

  17. #17
    D to the MAX gr8shot's Avatar
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    Re: Surging idle from a 350 TBI

    Thats what I've been doing with mine to get by until I can figure out what the heck is causing it. I got my multimeter today in the mail, so I'll be going to town on it in the next few days. If I figure anything out I'll be sure to pass it on for everyone here who's got the TBI woes.
    04 Chevy LB7 Dmax. Some go fast goodies from Husker Diesel

    SOLD!!!! 07.5 Chevy LMM Dmax DPF Deleted, EFI Live from Husker Diesel & more 448hp/834tq

    1992 Chevy 1500 ECSB 4x4 350TBI 320k miles

    '95 Chevy 2500 ECLB swapped in a 95 12v cummins, NV4500, 2wd

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    Big Daddy 1-8-4-3-6-5-7-2's Avatar
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    Re: Surging idle from a 350 TBI

    So, for now it's drivable. It will still have a surging idle and misses out real bad just off idle and when cruising through town at under 2,000 rpm. If I mash the gas it will go real good. I'm giving up for a few weeks on this thing.

    The old '95 I had was about to roll over 200,000 miles on the 4.3 with no problems at all except a TB gasket leak. The only symptom it had was a high idle. I drove it for a few months until it idled around 1,500 rpm and squealed continuously, then replaced the gasket and all was good.



    .
    :-)

    '96 Suburban, 17x9 Helo's, 37" Toyo MT's, 14 bolt sf, 5.13 Yukon by Custom Differentials, Black Bear tune!



  19. #19
    D to the MAX gr8shot's Avatar
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    Re: Surging idle from a 350 TBI

    Well I got mine fixed. Timing was off by 20 degrees. Since you said you've checked yours already then it's probably not much help to you. But hopefully you can get the thing figured out!!
    04 Chevy LB7 Dmax. Some go fast goodies from Husker Diesel

    SOLD!!!! 07.5 Chevy LMM Dmax DPF Deleted, EFI Live from Husker Diesel & more 448hp/834tq

    1992 Chevy 1500 ECSB 4x4 350TBI 320k miles

    '95 Chevy 2500 ECLB swapped in a 95 12v cummins, NV4500, 2wd

  20. #20
    Big Daddy 1-8-4-3-6-5-7-2's Avatar
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    Re: Surging idle from a 350 TBI

    Congrats on getting yours fixed. I think I'm going to give up on mine for a while... it's starting to annoy me.
    :-)

    '96 Suburban, 17x9 Helo's, 37" Toyo MT's, 14 bolt sf, 5.13 Yukon by Custom Differentials, Black Bear tune!



 

 
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